This invention relates to memory devices including magnetic discs. A flat and circular disc coated with a magnetic recording material is a well known type of a memory device in the computing arts. In the case of a single disc mounted on a hub, it is often desirable to have interchangeability of a number of disc-hub assemblies on a given drive spindle. To insure convienience in changing discs, a permanent magnet mounted on the spindle is used to attract an armature plate mounted to the hub carrying the disc. Attraction between the magnet and armature plate maintains the hub against the spindle.
Due to the minute spacing between adjacent data tracks on the disc recording surface, extremely accurate initial centering and repeatability of centering are vital to to proper recording and reading on the disc. The accuracy is impaired if the magnetic force can cause the disc to be pulled to the spindle at the periphery before proper centering. One solution to this problem is a hub structure disclosed in U.S. Pat. No. 4,224,648 to Roling granted Sep. 23, 1980. The disc hub includes a centering element or ball, a peripheral collar, and a diaphragm between the element and collar which flexes to allow axial displacement of the collar relative to the centering ball. A cup in the spindle captures the ball whenever the hub is approximately centered on the spindle, and further guides the ball to the spindle center. After centering, the diaphragm flexes to allow the collar to move, in response to a magnet in the spindle, to a full seating against the spindle.
Such a hub, including the collar, diaphragm and a housing for the centering ball, is preferably an aluminum casting. The centering ball is then mounted into the casting and the armature, of a magnetizable material, is attached. To reduce cost, it is desirable to substitute a plastic for the aluminum casting. Not only is the material itself less expensive; post-casting machining operations, required for the aluminum die casting, can be eliminated using a more accurate plastic mold, which initially can form a hub nearer to the required tolerances.
Centering accuracy is diminished, however, when a plastic hub is substituted for an aluminum hub. The plastic, having a significantly lower elastic modulus, has proportionally less diaphragm stiffness in the radial direction to cause the collar to follow movement of the centering ball in spite of friction between the collar and spindle. The result is that in spite of the centering ball's centered position within the spindle cup, the armature and collar strain the diaphragm and become firmly planted off center on the spindle, causing the disc to be improperly centered. The result is a track run out of prerecorded signals on the disc proportional to the radial force in the diaphragm.
It is an object of the invention, therefore, to reduce the strain in the diaphragm and thereby maintain the geometry between the centering ball and collar. Another object of the invention is to reduce the frictional force between the hub and spindle as the hub is centered on the spindle. Yet another object of the invention is to produce a low cost hub capable of accurately centering a disc with respect to a drive spindle.